Mr Davison's interesting paper was, he says, suggested by a letter of mine published in Nature on February 6, 1879. In that letter it is pointed out that the stratum of the Earth where the rate of cooling is most rapid lies some miles below the Earth's surface. Commenting on this, I wrote:—

“The Rev. O. Fisher very justly remarks that the more rapid contraction of the internal than the external strata would cause a wrinkling of the surface, although he does not admit that this can be the sole cause of geological distortion. The fact that the region of maximum rate of cooling is so near to the surface recalls the interesting series of experiments recently made by M. Favre (Nature, Vol. XIX., p. 108), where all the phenomena of geological contortion were reproduced in a layer of clay placed on a stretched india-rubber membrane, which was afterwards allowed to contract. Does it not seem possible that Mr Fisher may have under-estimated the contractibility of rock in cooling, and that this is the sole cause of geological contortion?”

Mr Davison works out the suggestion, and gives precision to the general idea contained in the letter. He shows, however, that there is a layer of zero strain in the Earth's surface, and that this layer, instead of that of greatest cooling, must be taken to represent Favre's elastic membrane.